Pronouns were invented to avoid the necessity of repeating nouns. The section on Pronouns deals with this subject, and also with the device known as "the polite alias" or "elegant variation".
Unnecessary repetition of a word is irritating to a reader and distracts his attention. If it can be avoided in a natural way it should be. For instance, in the sentence "The Minister has considered this application, and considers that there should be a market in Canada", the repetition of "consider" gives the sentence a clumsy and careless air. The second one might just as well have been "thinks". It would have been easy also to avoid the ugly repetition of essential in the sentence "it is essential that the Minister should have before him outline programmes of essential works". But where the same thing or act is repeatedly mentioned, it is better to repeat a word than to avoid it in a laboured and obvious way.
Irritating repetition of a sound (assonance) is always mere carelessness.
Reverting to the subject of the letter the latter wrote ...(This is indefensible because it could so easily be avoided by calling "the latter" by name.)